Burning Fat Through Exercise

Fat is a national obsession. Virtually everyone is concerned about how “fat” they are. It’s the hottest topic in the medical literature and at scientific conferences. And government health agencies are spending tens of millions of dollars to find ways to reverse what has been called an epidemic of obesity.

All of this concern is for good reason. In the past 25 years, the number of cases of obesity, defined as more than 30 pounds overweight, in the United States has increased dramatically — from 12% of the population in 1990 to 35.7% between 2009 and 2010 — and another 40% of Americans are overweight, defined as up to 30 pounds overweight. The cost of diseases associated with obesity — including Type 2 diabetes, heart disease, high blood pressure, gallbladder disease, stroke, some types of cancer, and many other conditions — has been estimated at $190.2 billion per year. What’s more, there doesn’t seem to be any indication that this trend toward obesity will slow down or turn around any time soon.

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As much time as we spend talking and worrying about how much fat we eat, how much it clogs our arteries, and how the prevalence of obesity in America is skyrocketing, most people know surprisingly little about what fat is, how the human body stores it, or how the body can get rid of it.

What is fat?

First, a bit of chemistry, but nothing you can’t handle. Fat is an oily compound composed of the elements carbon, hydrogen, and oxygen. In the body, the carbon, hydrogen, and oxygen molecules bind together like links to form chains of fatty acids. When chains of fatty acids connect together with a molecule called glycerol, it’s called a glyceride. Triglycerides — three fatty acid chains connected to a glycerol molecule — are the main type of fat in the foods we eat and in our bodies.

Even though body fat would appear to just sit there on our hips, thighs, and abdomens, it actually serves many roles. It insulates us from the cold, pads and supports our vital organs, muscles, and bones, and is part of the structure of every cell membrane. It’s an active organ, too. Scientists have discovered that fat cells behave very much like endocrine glands, secreting many different substances. At last count, more than 27 different enzymes, hormones, and neurotransmitters have been identified that are secreted by fat cells. They are responsible for, among other functions, controlling and regulating appetite, blood pressure, some nervous system and hormonal signals, gene expression, and the formation of testosterone and some forms of estrogen. In short, fat cells are important organs with vital regulatory functions that help our bodies run smoothly.

Humans have anywhere from 25 billion to 275 billion fat cells stored in their bodies. The average person has around 35 billion. For the average man, fat accounts for about 22% of his body weight; for the average woman, 28%. (In comparison, a trained, male athlete has about 10% body fat, and an obese person has between 40% and 50% body fat.) Fat cells are microscopic, 10–20 times smaller than the diameter of a human hair (which is about 100 microns), but they are packed with energy. The average person has more than 100,000 calories stored as fat on his body, theoretically enough fuel to jog from New York City to Chicago.

Fat is stored in special compartments called adipocytes, which are located all over the body and are often referred to as fat cells. Your genes determine where fat accumulates on your body; you don’t have any control over it. Women tend to accumulate it in the adipocytes on the hips, buttocks, and thighs, and men tend to accumulate it in the abdomen. The reason for the different accumulation patterns between sexes has not been identified, but one thing is certain: The fat that accumulates in the abdomen is associated with a high risk for Type 2 diabetes, high blood pressure, cardiovascular disease, and other medical conditions. The fat that accumulates in the lower extremities, although it may not be cosmetically appealing for some, does not present any cardiovascular or metabolic health risk.

Men are generally at higher risk for cardiovascular disease than women, and one of the reasons may be their higher percentage of abdominal fat. The recommendation for a healthy body circumference measured around the belly button is less than 40 inches for men and less than 35 inches for women. If your abdomen is larger than that, your risk for cardiovascular disease and other conditions increases.

Where does fat come from?

Fat comes from foods we eat, particularly animal and dairy products. Every time you eat a food with fat, the fat gets digested by bile and pancreatic enzymes in the small intestine. The resulting fatty acids are then absorbed and transported to the liver, where essential nutrients are removed. Then the rest is wrapped up and packaged together with proteins to form chylomicrons, so that it can travel through the bloodstream to all the organs and cells that need it.

If you eat more fat than your body needs, it either gets stored in the arteries of your heart (which is not a good thing), in your liver (which can give you a fatty liver and is also not a good thing), or in the adipocytes on your hips, thighs, buttocks, or abdomen (another not-so-good thing). Adipocytes love to store fat. They gobble it up like there’s no tomorrow, and with the ample amount of inexpensive, fatty foods available in our society and the copious amounts many of us eat, most adipocytes in the United States are well-fed, happy buckaroos.

When you fill an adipocyte with fat, it gets larger, just as a balloon gets larger when you fill it with air. Contrary to popular belief, however, when adipocytes get too big, they do not divide and multiply to form more adipocytes. Instead, the adipocyte grows like an overinflated balloon until it reaches a maximum diameter of approximately 15–20 microns, then it sends a biochemical signal to adjacent, immature preadipocytes in the body to begin storing the extra fat. These preadipocytes then grow larger, too, and when they become fully mature, plump adipocytes, you gain more body fat. The larger your adipocytes become, the higher your percent body fat will be.

During childhood and into puberty, both the number of mature adipocytes and the size of the adipocytes increases. It is believed by many scientists that the number of adipocytes stays fairly stable after adolescence and that instead of an increase in the number of adipocytes during adulthood, the mature adipocytes enlarge in size, stimulating the preadipocytes to get bigger too. There is some recent evidence to suggest that the number of adipocytes can also increase during adulthood, but whether we increase our body fat through increases in the number or the size of the adipocytes, one thing is certain: Excess fat intake leads to increases in body fat.

It’s not just fatty foods that make us fat, however. Whenever we eat excess calories, we stand a good chance of gaining fat. Excess carbohydrate that isn’t used by the body may be converted to fat, and then it too gets stored in the adipocytes. However, some very interesting research has suggested that not as much carbohydrate as was once thought gets converted to fat. Scientists are looking carefully at the role of carbohydrate and its role in making people gain excess fat, but for now, it’s believed that excess calories in any form causes fat gain.

Burning fat

Remember that fat cells are active glands. Not only do they secrete important proteins, enzymes, and hormones, but they also secrete fat. They secrete it because it’s a valuable fuel for muscles. It’s packed with energy, providing nine calories per gram, whereas a gram of glucose contains only four calories. A calorie is a unit of energy contained in our food that provides fuel for our muscles, just like gasoline provides fuel for an automobile. Think of muscle as the engine in your body, and think of fat as high-test fuel, providing more than twice the amount of calories or energy as a gram of glucose. This means that you can go more than twice as far on a gram of fat as you can on a gram of carbohydrate. The catch is learning how to get the fat to the muscle so that it can be burned for energy.

Whenever you move, whether you’re doing an endurance exercise like running or just taking a walk around the block, your muscles need fuel to burn. The fuels that muscles use are glucose and fat. If you’ve ever checked your blood glucose level before and after a bout of exercise or a brisk walk, you know that it usually drops, because the muscles use the glucose in the blood for fuel. Fat is used for fuel by the muscles in much the same way. Here’s how it works, step by step:

1. You take a brisk walk.

2. In response to the movement, certain hormones, including epinephrine (adrenaline) and norepinephrine, get released from the adrenal glands, and the blood levels of these hormones start to rise.

3. The hormones circulate around the body and attach to special hormone receptors on the adipocytes.

4. Once attached to the receptors on the adipocytes, a complex chain of biochemical events takes place in the cell, signaling the adipocyte that the body needs fuel.

5. Being the clever devil that it is, the adipocyte receives the signal and processes the stored fat, mobilizes it, and sends it out of storage and into the bloodstream.

6. Once in the bloodstream, the fat gets wrapped up again in chylomicrons so that it can travel in the bloodstream to the muscles that need it.

7. Once the package of fat and protein arrives at the muscle, another complex chain of events occurs, stimulated by an enzyme called lipoprotein lipase and, lo and behold, the fat enters the muscle.

8. Once in the muscle, a truly biochemical magic act occurs. What happens is that fat gets broken down to its smallest elements (carbon, hydrogen, and oxygen), and inside a part of the muscle cell called the mitochondria, known as the power house of the cell, enormous amounts of energy are produced, which allow the muscles to move.

Dozens of complex biochemical reactions have to occur to produce the energy for our muscles to move, but the good news is that you don’t have to remember how to do any of it. All you have to do is move. The body takes care of all the rest.

The physical activity recommendation for improving health is to accumulate 30 minutes of moderate-intensity physical activity on most, if not all, days of the week. The key words in this recommendation are “accumulate,” which means you can do it in three bouts of 10 minutes, two bouts of 15 minutes, or one bout of 30 minutes, and “moderate-intensity,” which means the work should leave you feeling warm and slightly out of breath but not exhausted. Walking, biking, dancing, weight lifting, swimming, climbing stairs, hiking, gardening, housework, and many other activities of daily living such as walking your dog, washing your car by hand, and mowing your lawn with a push mower will all do the trick. In most studies of physical activity, even modest amounts of activity help the body reduce fat, if not all over the body, then certainly in the abdomen and deeper in the visceral fat where it counts most for good health.

Building muscle — through weight lifting or other resistance exercises — will help, too. Muscle burns calories and helps you maintain your metabolic rate. The more muscle you have, the bigger your body’s engine, and the more likely you will be to burn fat.

Once adipocytes get the signal from hormones and release fat into the bloodstream, they shrink just like a balloon that you let air out of. When they shrink, so does your body fat. But if you eat excess fat once you’ve shrunk your adipocytes, chances are it will find its way right back to the adipocyte, and once again you’ll gain fat.

No matter how much physical activity you do, adipocytes never shrink so much that they disappear entirely. Like a balloon that you let all the air out of, you’re always left with some remnant. The only way to totally remove adipocytes from your body is with a surgical procedure such as liposuction or excision. But even with these procedures, if you go back to eating excess fat, you’ll put all the fat back on.

Spot reduction

Many people subscribe to the idea that you can do a specific exercise for a specific body part and the fat will come off that particular area first. For instance, some people think that if they do sit-ups they will lose abdominal fat. However, that’s not the way it works.

Fat on your body belongs to all of your body, and when you start to move, you have no control over which adipocytes release fat first. When you take a brisk walk, for instance, the muscles in your legs may be burning fat that was released from an adipocyte in your gut, or from your face, from your legs or from within the leg muscle itself. And it’s usually a consistent pattern. Everyone who has ever lost and regained weight more than once can tell you that the pattern of their weight loss is almost always the same. Many people lose fat from their face first and from their hips, thighs, and buttocks last. Genetics probably determines the pattern of weight loss and, of course, we have no control over our genes. However, we can shape and tone our physiques by building muscle through exercise.

The other good news is that abdominal fat, particularly the deeper abdominal fat known as visceral fat, which is implicated in heart disease, insulin resistance, and other medical conditions, is known to be very lipolytic. What that means is that abdominal fat is very sensitive to physical activity and readily releases fat in response to movement. That’s good news because it means that a relatively small amount of activity can have a significant impact on your health.

In one study of 24 people with Type 2 diabetes who exercised for 45 minutes three times per week for 8 weeks, visceral fat and subcutaneous abdominal fat — the fat just below the skin — was reduced significantly. Moreover, insulin resistance improved by 46%. Most interesting was that the subjects lost significant amounts of abdominal fat and improved their insulin sensitivity even though they lost very little body weight.

What about cellulite?

The lumpy fat that is often called cellulite is nothing special; it’s just ordinary fat. Its appearance is caused primarily by irregular patterns of connective tissue beneath the skin. What happens is that the adipocytes, which form in compartments in the shape of little honeycombs, get pushed into the skin by the irregular connective tissue, causing the dimpling that is characteristic of cellulite. Studies have shown that people who have cellulite have different patterns of connective tissue than people who don’t, and men tend to have this irregular pattern much less often than women.

Cellulite is not directly a function of excess weight, but rather a genetic difference in the way adipose tissue and connective tissue form. In fact, cellulite occurs in people who are lean as well as in those who are overweight. Skin creams sold to reduce cellulite make the skin swell so that the appearance of the cellulite changes, but the effects are short-lived, and the creams do nothing to change the structure of the connective tissue. There is no health risk to cellulite, and although weight loss and exercise can have some effect on it, in most cases, physical activity does not significantly change its appearance.

The bottom line

The bottom line to losing weight and fat is that you must burn more calories than you consume. If you eat 2,000 calories a day and only burn 1,500, you’re going to gain weight. On the other hand, if you consume 1,500 calories and burn 2,000, you’ll be in caloric deficit by 500 calories. Since it takes 3,500 excess calories to gain a pound, you’d lose one pound per week if you produced a 500-calorie deficit each day of the week. To do that, you could reduce your calorie intake by 250 per day and increase your physical activity by 250 calories per day (for a 150-pound person, a 2.5-mile walk is all it would take). Do that each and every day of the week and you’ll drop a pound per week.

Fat isn’t so bad if we manage it. We need it to survive, but we don’t need lots of it. The average person has more than enough stored fat on his or her body to sustain life for weeks. What we need to do is keep an eye on how much fat we eat (your dietitian can help you with that) and be physically active. No other intervention will do more to burn fat and improve your health than physical activity. Remember that you don’t have to do Herculean amounts of activity. If performed regularly and at a moderate intensity, just about any type of physical activity will help you burn fat, achieve good health, improve your diabetes control, and perhaps most important of all, improve the quality of your life. There’s no time like the present to get started. Results are just around the corner.

Richard Weil is an exercise physiologist and a consultant to St. Luke's-Roosevelt Hospital Center in New York City.

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